The present invention relates to a catalytically effective composition for a multilayer catalyst for exhaust gas after-treatment of combustion facilities and to a multilayer catalyst comprising the catalytically effective composition. Moreover, the invention relates to an exhaust gas after-treatment system and to a vehicle which both comprise the catalyst according to the invention.
It has long been customary, especially with regard to motor vehicles, to subject the exhaust gas of a combustion motor to after-treatment using a catalyst. The task of the catalyst is to convert the pollutants generated during combustion, i.e., hydrocarbons (CmHn), carbon monoxide (CO), and nitrogen oxides (NOx), into the non-toxic substances carbon dioxide (CO2), water (H2O), and nitrogen (N2). The following oxidation and reduction reactions take place in this process:2CO+O2→2CO2 2C2H6+7O2→4CO2+6H2O2NO+2CO→N2+2CO2 
There are various types of catalysts. The best-known, aside from the three-way catalyst, are oxidation catalysts and NOx storage catalysts.
The three-way catalyst, also referred to as a controlled catalyst or “G-Kat,” has become standard equipment in a motor vehicle fitted with a combustion engine. In this context, the term “controlled” refers to the motor management of the combustion. The three-way catalyst can only be used in vehicles equipped with a combustion engine and lambda control. In a three-way catalyst, the oxidation of CO and HmCn and the reduction of NOx take place in parallel. This requires a constant air-fuel mixture at a stoichiometric ratio of lambda (λ) equal to 1.
In a combustion engine, the lambda probe ensures controlled combustion of the fuel. The lambda probe is used to determine the air-fuel ratio in the exhaust gas of the combustion engine. The measurement is based on the residual oxygen content present in the exhaust gas. The lambda probe is the main sensor in the control loop of the lambda control for catalytic after-treatment with a controlled catalyst and supplies the measured value to the motor control unit.
The lambda control establishes a desired lambda value in the exhaust gas of a combustion engine. In this context, lambda denotes the air-fuel ratio, which is the ratio of the mass of air available for combustion to the minimal stoichiometric mass of air required for complete combustion of the fuel. At the stoichiometric fuel ratio, exactly the amount of air required for complete combustion of the fuel is present. This is called λ=1. If more fuel is present, the mixture is called rich (λ<1), whereas an excess of air being present corresponds to a lean mixture (λ>1). If there is any deviation from the stoichiometric air-fuel ratio towards an excess of air, i.e., lean region, not all nitrogen oxides are decomposed, since the requisite reducing agents are being oxidized earlier. In the rich region, i.e., air deficit, not all hydrocarbons and not all of the carbon monoxide are decomposed.
The air-fuel equivalence ratio lambda, also called “air excess,” air excess number,” or “air ratio” for short, is a parameter of combustion technology. This parameter provides some feedback concerning the progress of the combustion, temperatures, generation of pollutants, and the efficiency. Proper fine-tuning of carburetor or fuel injection facility, and thus the adjustment of lambda, has a major impact on motor performance, fuel consumption, and the emission of pollutants.
Combustion engines are usually controlled to a narrow range of approx. 0.97<λ<1.03. The range within these thresholds is called the lambda window. The best reduction of all three types of pollutants is attained within this window. At high motor performance, operating the engine with a rich mixture, and therefore colder exhaust gas, prevents the exhaust components, such as manifold, turbo-charger, and catalyst, from overheating.
To attain a value of λ=1 in operation, sufficient oxygen must be available in the catalyst in order to carry out the oxidation-reduction reactions indicated above. On the other hand, oxygen released during the reduction must be bound for the reduction of the nitrogen oxides to nitrogen to take place. Three-way catalysts usually contain an oxygen reservoir that is charged with oxygen at oxidizing conditions and can release oxygen again at reducing conditions.
In addition to the oxygen reservoir, a catalyst often also comprises at least one noble metal; usually this will be platinum, palladium, and/or rhodium. If aluminum oxide is also used in a catalyst, it is important to ensure that the rhodium does not become applied onto the aluminum oxide. At elevated temperatures, the rhodium adsorbs to the porous structure of the aluminum oxide and is therefore no longer available for the actual catalytic reaction. Accordingly, EP 1053779 A1 describes a catalyst in which the catalytically active layer comprises a cerium complex oxide and a zirconium complex oxide. While palladium is situated on the cerium complex oxide, platinum and rhodium are applied onto the zirconium complex oxide.
DE 10024 994 A1 describes a catalyst in which the noble metals are applied onto a substrate as separate layers. The catalyst comprises a first coating layer formed on a heat-resistant substrate and a second coating layer formed on the first coating layer. The first coating layer contains aluminum oxide bearing palladium; the second coating layer contains cerium zirconium complex oxides bearing both platinum and rhodium.
For improvement of the decomposition of exhaust gases in a catalyst, WO 98/09726 A1 describes a coating for a catalyst which comprises a first substrate for a first noble metal component and a second substrate for a second noble metal component, in which the average particle size of the second substrate is larger than the average particle size of the first substrate. This causes different noble metals to be separated from each other in operation of the catalyst. For this purpose, the respective noble metal components are affixed on their substrates and then ground to the desired size. The frits thus obtained are then applied onto a substrate to obtain a layer which comprises the smaller particles, in particular, in the lower region and the larger particles, in particular, in the upper region.
Different size distributions in a catalytically active layer are also known from EP 0556554 A2. Here, the coating dispersion that can be applied onto a catalyst comprises solids that have a multi-modal grain size distribution with different grain fractions.
Especially in motorcycles, the fluctuation of λ in operation of the motor can go beyond the common range for petrol engines of 0.97<λ<1.03. It is necessary in this case to have the catalyst still work properly and convert exhaust gases accordingly even if the deviation from λ=1 is larger, in particular in the range of 0.8<λ<1.2.
Accordingly, there is a need for catalytically effective compositions that can compensate even for high fluctuations of lambda in the range of 0.8<λ<1.2. Moreover, said compositions, used in a catalyst, are to produce high conversion rates in the treatment of exhaust gases. Specifically the emission of CO, HC, NOx, and CO2 should be reduced as compared to known catalysts both in rich and in lean operation of the motor.